damping factor in amplifiers

I recently herd someone speak about damping factor as it relates to audio amplifiers, is it something that can be controlled or needs to be a specific level?

Typically the Lower the Better, but it is a Function of the actual design.

Hi,

I always thought: the higher the better.

--> audioguru surely gives a detailed answer ;-)

Klaus

Yes, Sorry about that.
What I was really meaning to say is the Lower the Output Impedance of the Amplifier, the better the driving power, Especially on Transistor Amplifiers.

KlausST said:

Hi,

I always thought: the higher the better.

--> audioguru surely gives a detailed answer ;-)

Klaus

Click to expand...

I was looking forward to audioguru's reply no doubt, but from the time I have been reading about amplifiers I only herd about that yesterday and it wasn't from reading someone had to mention it.

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Is it possible to test the output impedance of an amplifier to have an idea what the damping factor would be.

Not Easily!

You can measure the output impedance of an amplifier fairly easily.

Short the signal input to ground, so the output of the amplifier should then (theoretically) be held at zero volts.

Use a signal source, such as another amplifier with a load resistor in series, to drive a reasonably high ac current back into the output of the amplifier being tested.

Whatever voltage you can measure across the output of the amplifier being tested is mostly due to that amplifiers output impedance. The internal amplifier feedback will try to hold the output at zero volts, and keep the output impedance very low.

But you should be able to measure the output impedance, just using ohms law.

The problem might be that the amplifier being tested may have some residual noise and dc offset that needs to be considered, but the method should still be workable.

A good amplifier has a damping factor of at least 3000 at low frequencies where a speaker resonates. Then the output impedance of the amplifier is only 8/3000= 0.002666 ohms which is almost impossible to measure. A speaker would not dare to even hint at resonating. Note that the resistance of the speaker coil and of the connecting wires are in series and reduce the effect of a very high damping factor but it still works. Here is a good article about it:

If the output impedance is truly only 2.6 milliohms, you should still be able to measure it by feeding (say) ten amps rms into the output, and being able to see 26mV rms of that above any noise on an attached oscilloscope.

Of you feed an AC current into the output of an amplifier to measure its impedance then it must try to produce the same amount of current to cancel it. 10A RMS into an 8 ohm speaker is 800W continuously! Not many audio amplifiers can do it.

True, it was only an example of the method.

But not many very low power amplifiers would have an actual measured output impedance as low as 2.6 milliohms.

That sort of resistance is like a 75mm length of 1mm squared ten amp rated twin core cable. Its pretty low.

OK so when building an amplifier does one consider damping factor, since the higher it is the better, is there a way to increase it which section of an amplifier would control that function.

Basically the negative feedback around the amplifier reduces the output impedance.
At least in theory, the more feedback, the lower the impedance becomes, and the better the damping factor.

But you are still limited by the wires connecting the amplifier to the actual loudspeaker.

Suppose you run 10 amp rated wires to your speakers, every meter of wire has about 70 milliohms of total resistance. And that would support about 800 watts of power to one loudspeaker, as Mr Audioguru has already pointed out..

Its pointless having an amplifier with 2 milliohms of output impedance, then running the output through cable that adds 200 milliohms of resistance to that.

So in other words your saying it's best to have a short cable run from your amplifier to the speakers?

Not all the time that can be avoided though

If damping factor is "your thing" then fatter shorter cables will probably help more than much you can probably do to the amplifier (in most cases).

But you need to actually start measuring things to know where you are, and what may or may not improve things.

Hi,

I'd like to add that it is a complex impedance.
So don't be surprised that you get different mOhms values with different frequencies.

I'd expect higher output impedance with higher frequencies = lower damping factor.

Klaus

The damping factor of an amplifier damps speaker resonance. Thump the cone of a woofer that is not connected to anything and it makes a BONG sound. Then thump the cone when it is shorted or is connected to an amplifier that has a high damping factor and it makes a THUMP sound. Speakers also resonate at higher frequencies.

If you design a cheap poor spec's amplifier that has no negative feedback then its distortion is very high and its and damping factor is low.
But if you design an amplifier that has a lot of negative feedback then its distortion is very low and its damping factor is very high.

Since a speaker has some resistance then adding 1 ohm of cable resistance in series with it will make only a small change in the effect of having a high damping factor. But don't use a mile of thin telephone wires (30 ohms).

Hi,

The damping factor of an amplifier damps speaker resonance.

Click to expand...

Audioguru, I hope you don´t mind a few words on that.

Damping means power dissipation. Power in the meaning of true power = V x I.
A large damping factor makes the voltage (caused by the output impedance of an amplifier) to become near zero.

--> your BONG and THUMP example:
If the voltage now is zero, then independent of currentn the dissipated power is zero, too.
An amplifier with high damping factor acts as a short circuit - when the speaker is excited.

Do you agree with me that the "THUMP" is the same when you short circuit the speaker wires without any amplifer?

****
In my eyes
* the power is mainly disipated in the ohmic resistance of the speaker coil. And it can only be dissipated when current flows. And enough current only can flow when the amplifier has a good damping factor.
I like amplifiers with high damping factor.
But I don´t like the phrase "damping factor" therefore I explain it differentely: I say an amplifier with high damping factor has better control of the speaker´s membrane movements.

Maybe just another expression for the same effect...

***
Resonance of a speaker:
Usually you don´t want a speaker to operate at or near it´s resonance frequency. Therefore one uses crossover in a loudspeaker box. It is low impedance (amplifier to speaker) in the pass band and it is high impedance at the stop band. But the resonance frequency is in the stop band. Ooops! Exactely where one needs the low impedance of the amplifier.. then there is the crossover at high impedance. It makes it impossible for the amplifier to control the membrane.
For sure there are expensive crossover circuits that take care of this. But only a few.

This is why I perfer active speakers, where there is no crossover between amplifier and speaker. The crossover is in front of the amplifier and each speaker in a box has it´s own amplifier.
... With high damping factor, for sure ;-)


Klaus

KlausST said:

Do you agree with me that the "THUMP" is the same when you short circuit the speaker wires without any amplifer?

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Yes, the resonance is damped so it is not audible.

Resonance of a speaker:
Usually you don't want a speaker to operate at or near it´s resonance frequency. Therefore one uses crossover in a loudspeaker box.

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I disagree. A woofer operates above, at and below its resonant frequency but performs poorly at high frequencies so the crossover network cuts high frequencies to the woofer.
A tweeter is damaged (too much cone excursion) if it is fed its resonant frequency or lower frequencies so the crossover network cuts low frequencies to the tweeter.

Rod Elliot's website about Elliot Sound Products is down so I can't copy what he says about active crossovers.

Hi,

I disagree. A woofer operates above, at and below its resonant frequency

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Yes, woofers. Indeed you are correct, as always.

***
Rod Elliot is no friend of active crossovers?

Do you know why?

Klaus